Interpretive Summary: Euphorbia spp. are predominant noxious weed species impacting rangelands throughout Europe and North America. Euphorbia esula L. (LS; Leafy spurge) specifically has threatened the northern Great Plains of North America since the early 1900’s. Euphorbia spp. in general have high nutritive value and are comparable to alfalfa and other regional forages; however, domesticated livestock and wildlife tend to avoid Euphorbia spp, especially larger ruminants such as cattle. Studies have shown that cattle foraging behavior is altered and utilization of non-infested rangeland increased due to under-utilization of moderate to high LS infested sites. This change in rangeland utilization creates an additional opportunity for encroachment of LS. Avoidance of LS by cattle is considered to be related to aversion-eliciting condensed tannins and diterpenoids (i.e. secondary compounds) that create ruminal or post-ruminal digestive imbalances. Adaptation to secondary compounds has been suggested to increase tolerance by ruminants that are gradually introduced to these compounds. Studies have observed no differences for in vitro DM or NDF disappearance when ewes received a diet containing 50% LS and 50% grass hay. Therefore, objectives for the present study were to evaluate fermentation characteristics of LS using in vitro dry matter disappearance (IVDMD) and gas production experiments. Fermentation substrates for IVDMD and gas production experiments were incremental increases in LS and inoculum donated by bovine and ovine ruminally-cannulated female animals while either consuming or not consuming LS in their diet. Our hypothesis was that by including LS in the diet of animals providing inoculum for the aforementioned in vitro experiments would result in an increase in substrate digestibility and gas production as LS increased in the fermentation substrate. In conclusion, it is clear that differences exist between species and previous inoculum exposure to LS. Euphorbia esula L. consumption alters rumen ecology that may favor some by-product production of fermentation while inhibiting that of other. The rumen is a complex ecosystem that relies not only on the consumption of dietary feeds, but the synchrony of by-products to be used by other rumen microorganisms and the host animal. The presence of condensed tannins, terpenoids, and other plant derived secondary compounds greatly influence the rumen ecosystem and subsequently animal performance and the desire to consume Euphorbia spp.

Technical Abstract:
Euphorbia esula L. (leafy spurge; LS) is indigenous to Eurasia and has negatively impacted rangelands in the northern Great Plains and Intermountain West of the United States as well as southern Canada. Our objectives were to evaluate the effect of increasing substrate concentrations of LS by in vitro digestibility and gas production using ruminal bovine and ovine inoculum with or without LS inclusion in the diet of donor animals. Two experiments were conducted evaluating diets with or without LS using two ruminally-cannulated cows and ewes as rumen inoculum donors. In experiment 1, animals were fed a diet of 15% LS (21.9% CP, 48% NDF, DM basis) and 85% barley hay (12% CP, 55.4% NDF, DM basis) based on previous day intake; whereas experiment 2, consisted of animals fed exclusively a barley hay diet. Substrate concentrations for in vitro disappearance and gas production consisted of BH: LS ratios including 10% (DM basis) increments of LS for a total of 11 substrate combinations. A 47% increase (P < 0.0001) for 48-h in vitro DMD (g/kg) and a 8% lower (P < 0.0001) 96-h gas production DMD was observed for cattle vs. sheep inoculum when donors consumed LS in their diets Additionally, a 5% increase (P = 0.006) for 48-h in vitro DMD and a 17% greater (P < 0.0001) 96-h gas production DMD was observed for cattle vs. sheep inoculum when donors were not consuming LS in their diets. Ruminal ammonia concentrations were lower when inoculum was incubated from donors consuming LS; however, greater DMD were concurrently observed. These findings suggest an adaptation period to LS may benefit rumen fermentation. The present study shows that differences exist between bovine and ovine rumen inoculum and that differences are most likely attributed to changes in the rumen microbial environment. An adaptation period may be necessary prior to grazing rangelands infested with LS for ruminants. Adaptation may be most beneficial for the bovine animal and encompass changes in rumen microbial populations and process in which microbial metabolism occurs.